Unlike most penguin species, where both the males and the females take turns to incubate the eggs, for emperors it’s the males that take charge for the whole of the incubation period over the Antarctic winter. In order to survive the frigid conditions, the males huddle together in large groups.

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Researchers interested in how these groups remain closely knit have attached temperature and light sensors to individual birds, but no one had looked at the dynamics of a huddle as a whole.

They found that when one bird steps, it triggers the coordinated movement of its neighbours in a wave-like motion.

“We had no idea how to describe these waves,” Zitterbart says. “We wanted to know how and where they are triggered within a huddle, and how they propagate.”

To find out, the team have now applied a mathematical model often used to study road congestion. They found that the model can predict individual penguins’ step-by-step movement within the huddle – just like it predicts how cars inch forward in a traffic jam.

Perfectly insulated

The model also shows that rather than simply being caused by cold penguins pushing in, waves can originate from birds at many different spots in the huddle, as long as their steps exceed a 2-centimetre threshold distance. This is about twice the thickness of the penguin feather layer. “That means a perfectly compact huddle tries to maintain each bird’s maximum fluffiness and insulation,” Zitterbart says.

Waves that started in two different groups can merge, helping smaller huddles grow into large throngs of thousands of birds that can withstand temperatures as low as -50 °C and wind gusts of 200 kilometres per hour.

The team is also investigating why emperor penguins take such tiny, frequent steps in the first place. The males incubate their eggs by balancing them on the tops of their feet and covering them with a layer of feathery skin called the brood pouch. One guess is that this waddling helps penguins rotate the eggs, so that one face doesn’t remain exposed too long. This season, Zitterbart has given fake eggs to some of the penguins that don’t have offspring of their own. These fakes are fitted with accelerometers and other measuring devices to collect movement data.

“Ultimately we want to create a formula to predict the state of a penguin huddle depending on the time of year, the number of penguins, and how cold it is,” Zitterbart says.

“This is really the first time that a quantitative basis for the huddle movement of penguins has been introduced by looking at the positioning and velocity of individual birds,” says James Butler, a Harvard University physiologist with expertise in collective behaviour. “It’s an exciting mathematical foundation for studying this behaviour.”